1. Field of Invention
This invention relates to a process for the conversion of oxygenates to hydrocarbons using small pore molecular sieve catalysts. More particularly, this invention relates to a process for conversion of oxygenates to olefins using silicoaluminophosphate molecular sieve catalysts which have been incorporated with certain transition metals after the synthesis of the molecular sieve.
2. Background Art of the Invention
Olefins have traditionally been produced through the process of petroleum cracking. Because of the potential limited availability and high cost of petroleum sources, the cost of producing olefins from such petroleum sources has been steadily increasing. Light olefins such as ethylene serve as feeds for the production of numerous chemicals.
The search for alternative materials for the production of light olefins, such as ethylene, has led to the use of oxygenates such as alcohols, and more particularly to methanol and ethanol or their derivatives as feedstocks. These and other alcohols may be produced by fermentation or from synthesis gas. Synthesis gas can be produced from natural gas, petroleum liquids, carbonaceous materials including coal, recycled plastics, municipal wastes, or any organic material. Thus, alcohol and alcohol derivatives may provide non-petroleum based routes for hydrocarbon production.
It is well known in the prior art to convert oxygenates to olefins by contacting the oxygenate with various types of catalysts. Medium and large pore molecular sieve catalysts, such as borosilicate, ZSM-5, SAPO-11, and SAPO-5, may be used. For example, U.S. Pat. No. 4,292,458 teaches a process in which a crystalline borosilicate, is converted to the hydrogen form, ion-exchanged with Ni(NO.sub.3).sub.2 in water, washed, and then calcined to give a catalyst useful for conversion of methanol to ethylene and propylene (among other products). Similarly, U.S. Pat. No. 4,311,865 teaches the use of the medium pore zeolite, ZSM-5 (approximately 5.5 Angstroms) pore size of, which is ion-exchanged with cobalt, and then calcined to produce a catalyst which is then used to convert methanol to hydrocarbons (including olefins). Both of these processes use ion-exchange to add the metal to the medium pore molecular sieve. The following references also teach the process of using the large pore catalyst SAPO-5 (pore size of approximately 8.0 Angstroms), for conversion of methanol to olefin; however, in these instances, the nickel is incorporated during synthesis, rather than by the use of ion-exchange: N. Azuma, et al., Nickel(I) Location and Adsorbate Interactions in Nickel(II)-Exchanged Silicoaluminophosphate Type 5 As Determined by Electronic Spin Resonance and Electron Spin Echo Modual Spectroscopies, Journal of Physical Chemistry, Vol. 99, No. 17, pages 6670-6 (1995) and V. Mavrodinova et al., Effect of the Introduction of Ni(II)--On the Catalytic Properties of SAPO-5 Molecular Sieves, Zeolite Chemistry and Catalysis, Pages 295-302, Elsevier Science Publishers B. V. Amsterdam (1991).
Small pore catalysts such as SAPO-34, have been used to convert methanol to olefins, as described in an article by T. lnui, Structure-Reactivity Relationships in Methanol to Olefins Conversion in Various Microporous Crystalline Catalysts, Structure-Activity and Selectivity Relationships in Heterogensis Catalysts, pages 233-42, Elsevier Science Publishers B. V. Amsterdam (1991). However, the conversion stability is not as good as when using medium pore molecular sieves which have been ion-exchanged with the metals. Based on the favorable effect of metal addition to medium pore molecular sieve, it would seem that this same effect would be seen using small pore molecular sieves. However, until now, as taught by Inui, the metal ion had to be incorporated into the catalyst during synthesis, rather than by post-synthesis ion exchange.
Inui has confirmed that nickel substitution into SAPO-34 during the synthesis process results in improving the selectivity of methanol to ethylene. Inui's experiments tested three different SAPO-34 catalysts for use in the conversion process. In the first, the catalyst without any nickel substitution was used as a comparative sample. In the second and the third, nickel was substituted during the synthesis for a resulting silicon to nickel ratio of 100 and 40 respectively. In all three experiments, a feed of 20% methanol and 80% nitrogen diluent was used. The reactions were carried out at a total pressure of one atmosphere (0.1 MPa), a temperature of 450.degree. C., and a gas hourly space velocity (GHSV) of 2,000 hr.sup.-1.
The experiments teach that the ethylene yield will increase from 30% to 60% by using the Ni-SAPO-34 catalyst with the Si/Ni ratio of 100, as compared to the untreated SAPO-34 catalyst. Inui's use of the Ni-SAPO-34 catalyst with the Si/Ni ratio of 40 increased the ethylene yield from 30% to 90% as compared to the untreated SAPO-34 catalyst. The combined ethylene and proplyene yield was also increased on an absolute basis by 25% and 34%, respectively.
One can see that the nickel incorporation has a definite impact upon the ethylene and propylenes yields. However, even though Inui has demonstrated that nickel substitution is attractive, the nickel was substituted during the catalyst synthesis process. Metal substitution via catalyst synthesis generally requires elevated temperatures, elevated pressures, and special equipment, therefore making it commercially less attractive. In contrast, post-synthesis metal incorporation can be carried out under milder conditions. In addition, the physical characteristics, such as particle size, can be varied prior to metal incorporation to achieve greater flexibility, thus allowing a wider range of operating parameters with which to achieve the incorporation.
Therefore, in view of the problems associated with incorporating the metal during synthesis, it would be commercially useful and desirable to be able to produce such a catalyst by incorporating after molecular sieve synthesis, rather than during synthesis. A post-synthesis technique would provide flexibility in catalyst preparation, choice of metal additive, metal concentration, and selection of molecular sieves.